Principally LiCoO2 has been used as a positive active material for lithium secondary batteries. However, the discharge capacity is about 140 to 150 mAh/g.
Materials are known in which LiCoO2 forms a solid solution with other compounds. Li[Co1−2xNixMnx]O2 (0<x≦½), a solid solution having an α-NaFeO2 crystal structure and formed of three components: LiCoO2, LiNiO2 and LiMnO2, was published in 2001. Lithium secondary batteries using, as an active material, LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2 that is one example of the aforementioned solid solution have a discharge capacity of 150 to 180 mAh/g, which is superior to that of lithium secondary batteries using LiCoO2.
Non-Patent Documents 1 to 4 each propose a solid solution having an α-NaFeO2 crystal structure and formed of three components: Li[Li1/3Mn2/3]O2, LiNi1/2Mn1/2O2 and LiCoO2. This material, as can be expressed as Li[Li, Mn, Ni, Co]O2, has Li, in addition to a transition metal, at a site where Co is present in LiCoO2 having an α-NaFeO2 crystal structure. Therefore, a higher discharge capacity can be expected, and Non-Patent Documents 1 to 4 each describe a discharge capacity of about 180 to 200 mAh/g.
However, as in Comparative Example described later, lithium secondary batteries having the above-mentioned solid solution as an active material have such a problem that although the discharge capacity (25° C.) is high, the irreversible capacity during initial charge-discharge is high, the initial charge-discharge efficiency (hereinafter, abbreviated as “initial efficiency”) is low, and the discharge capacity at a low temperature is low.
A large number of attempts to substitute with a different kind of element a part of the transition metal site of a transition metal compound used in a positive active material for a lithium secondary battery have been made, and it is needless to show examples in other active materials having a tetragonal spinel structure, such as LiMn2O4. However, the effect brought by substitution with a different kind of element varies among active materials, and needless to say, it is very difficult in the art to predict whether an effect exhibited in a different material is similarly exhibited in another material.
Non-Patent Document 5 describes that the initial efficiency during initial charge as a positive electrode, which originates from Li[Li0.2Mn0.54Ni0.13Co0.13]O2, can be improved by mixing V2O3 with Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (FIG. 2, FIG. 3).
Non-Patent Document 6 describes that the initial efficiency during initial charge-discharge is improved as a result of treating an active material: 0.3Li2MnO3.0.7LiNi0.5Mn0.5O2 with nitric acid (FIG. 4, FIG. 5).
Non-Patent Document 7 describes that the initial efficiency during initial charge-discharge is improved as a result of treating an active material: 0.3Li2MnO3.0.7LiNi0.5Mn0.5O2 with nitric acid, followed by further exposing the active material in an ammonia gas stream at 200° C. for 20 hours (Table 2, FIG. 5, FIG. 6).
Non-Patent Document 8 describes that by substituting with fluorine a part of oxygen forming an active material so that the active material is represented by Li (Li0.2Ni0.15+0.5zCo0.10Mn0.55−0.5z)O2−zFz (0≦z≦0.10), the impedance of a battery having the active material as a positive electrode and graphite as a negative electrode can be reduced (FIG. 5).
Patent Document 1 describes “a lithium/nickel/manganese/cobalt composite oxide of layered structure, which has a chemical composition of LiaNixMnyCozO2+b (x+y+z=1, 1.00<a<1.3, 0≦b<0.3), wherein the diffraction peak angles 2θ of the (003) plane and the (104) plane in the Miller index hkl in powder X-ray diffraction using a CuKα ray are 18.65° or more and 44.50° or more, respectively, the diffraction peak half widths thereof are both 0.18° or less, the diffraction peak angles 2θ of the (108) plane and the (110) plane are 64.40° or more and 65.15° or more, respectively, and the diffraction peak half widths thereof are both 0.18° or less” (claim 1), and describes that “if the diffraction peak angles 2θ of the (003) plane and the (104) plane in the Miller index hkl of powder X-ray diffraction using a CuKα ray are less than 18.65° and less than 44.50°, respectively, the phase interval decreases, so that diffusion of lithium ions are hindered, leading to deterioration of a charge-discharge characteristic; and if the diffraction peak half widths of these planes are both larger than 0.18°, the charge-discharge characteristic is deteriorated because of insufficient growth of crystals or large variations in composition” (paragraph [0017]).
Patent Document 2 describes the invention of “a positive active material having as a main component a composite oxide represented by Li[MncNidCoeLiaM″b]O2 (where M″ is at least one element selected from the group consisting of B, Mg, Al, Ti, V, Cr, Fe, Cu and Zn, d≦c+e+a+b, c+d+e+a+b=1, 0≦a≦0.05, 0≦b≦0.05, 0.2≦c≦0.5, 0.02≦e≦0.4), wherein the specific surface area is 0.3 m2/g to 1.5 m2/g (inclusive) as measured by the BET method, the positive active material has an X-ray diffraction pattern attributable to a space group R3/m, the relative intensity ratio of a diffraction peak at 2θ=44.1±1° to a diffraction peak at 2θ=18.6±1° is 0.6 to 1.1 (inclusive), the half width of a diffraction peak at 2θ=18.6±1° is 0.13° to 0.20° (inclusive), the half width of a diffraction peak at 2θ=44.1±1° is 0.10° to 0.17° (inclusive), and the particle diameter is 3 μm to 20 μm (inclusive)“(claim 6), and also shows that a lithium secondary battery using the active material can have both an excellent high rate discharge characteristic and excellent charge-discharge cycle performance.
Patent Document 3 describes “a positive active material containing a composite oxide represented by LiaMn0.5−xNi0.5−yMx+yO2 (where 0<a<1.3, −0.1≦x−y≦0.1, M is an element other than Li, Mn and Ni)” (claim 1), and also describes that “the positive active material according to claim 2 is characterized in that the M is at least one element selected from the group consisting of Al, Mg and Co, and the positive active material contains a composite oxide in which the coefficients in the composition formula satisfy the following formula: 0.05≦x<0.3, 0.05≦y<0.3, −0.1≦x−y≦0.02, 0<a<1.3 and x+y<0.5; and according to this configuration, a positive active material, which allows production of a nonaqueous electrolyte secondary battery particularly being excellent in high rate discharge performance and charge-discharge cycle performance and having a high energy density, can be obtained” (page 6, line 7 from the bottom to page 7, line 4). However, it is described that “the positive active material according to claim 7 is characterized in that the half width of the diffraction peak at 2θ: 18.6±1° is 0.05° to 0.20° (inclusive), and the half width of the diffraction peak at 2θ: 44.1±1° is 0.10° to 0.20° (inclusive); and according to this configuration, a positive active material, which allows production of a nonaqueous electrolyte secondary battery particularly having a high energy density (high discharge capacity) and being excellent in charge-discharge cycle performance, can be obtained” (page 9, line 11 to line 16).
Patent Document 4 describes the invention of “an active material for a lithium secondary battery comprising a solid solution of a lithium transition metal composite oxide having an α-NaFeO2 crystal structure, wherein the composition ratio of Li, Co, Ni and Mn contained in the solid solution satisfies Li1+1/3xCO1−x−yNiy/2Mn2x/3+y/2 (x+y≦1, 0≦y, 1−x−y=z), (x, y, z) is represented by a value present on the line of or within a heptagon ABCDEFG having point A (0.45, 0.55, 0), point B (0.63, 0.37, 0), point C (0.7, 0.25, 0.05), point D (0.67, 0.18, 0.15), point E (0.75, 0, 0.25), point F (0.55, 0, 0.45) and point G (0.45, 0.2, 0.35) as apexes, in a Li[Li1/3Mn2/3]O2(x)-LiNi1/2Mn1/2O2(y)-LiCoO2(z)-system triangular phase diagram, and the intensity ratio of the diffraction peak of the (003) plane and the (104) plane in X-ray diffraction measurement is I(003)/I(104)≦1.56 before charge-discharge, and I(003)/I(104)>1 at the end of discharge” (claim 1).
Patent Document 5 describes the invention of “a positive active material comprising lithium-containing metal composite oxide particles, wherein the half width of an X-ray diffraction peak, which can be attributed to a space group R-3m and corresponds to the (104) plane, is within a range of 0.06 to 0.15°, and the average of shape coefficients SF1 calculated by the following formula (1) is more than 1 and equal to or less than 3.3 . . . ” (claim 1), and also shows that the crystal structure of lithium-containing metal composite oxide particles is attributed to a space group R-3m, and a high discharge load characteristic (high rate discharge characteristic) is obtained when the half width of an X-ray diffraction peak corresponding to the (104) plane is within a range of 0.06 to 0.15°, and if the half width is more than 0.15°, the crystallinity of the lithium-containing metal composite oxide is reduced, so that it is difficult to obtain a high rate discharge characteristic (paragraph [0025]).
Patent Document 6 describes that “an additional function can be exhibited by doping LiNi1/3Mn1/3Co1/3O2 of the invention with a different kind of element . . . ” (paragraph [0077]), and also shows that an oxide represented by Li[Lix(Ni1/3Mn1/3Co1/3)1−x]O2 (where 0≦x≦0.3), which has an increased lithium atom ratio, can be used, and a nickel manganese cobalt composite oxide prepared by mixing a composite oxide obtained in coprecipitation and lithium hydroxide in a dry process, and calcinating the mixture at 1000° C. is a hexagonal crystal system that belongs to a layer structure R-3m (paragraphs [0028] to [0030]).
Patent Document 7 describes the invention of “a lithium nickel manganese cobalt-based composite oxide powder for a lithium secondary battery positive electrode material, which comprises a crystal structure attributed to a layered structure and the composition of which is represented by the following formula (I):Li[Liz/(2+z){LixNi(1−3x)/2Mn(1+x)/2)(1−y)Coy}2/(2+z)]O2  (I)(where 0.01≦x≦0.15, 0≦y≦0.35, 0.02(1−y)(1−3x)≦z≦0.15(1−y)(1−3x))” (claim 6), and shows that it is important that the composite oxide is slightly rich in Li amount as compared to the stoichiometric composition, whereby battery performance (particularly rate characteristic and power characteristic) is improved (paragraphs [0014] and [0015]).
Patent Document 8 describes the invention of “a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive active material formed of a lithium-containing oxide, a negative electrode, and a nonaqueous electrolyte, wherein the lithium-containing oxide contains LiANaBMnxCoyO2±α (where 0.5≦A≦1.2, 0<B≦0.01, 0.40≦x≦0.55, 0.40≦y≦0.55, 0.80≦x+y≦1.10 and 0≦α≦0.3) that belongs to a space group P63mc and/or a space group Cmca” (claim 1), and shows that by having this composition to thereby increase the charge potential, the crystal structure becomes hard to be collapsed even though a large amount of Li is drawn off (paragraph 0069), so that a high initial charge-discharge efficiency, a high charge-discharge capacity and good cycle performance are obtained. (Paragraph 0133)
Patent Document 9 describes the invention of “a positive active material for a lithium secondary battery, which comprises a layered rock-salt-type lithium composite oxide that belongs to a space group of R-3m, wherein the lithium composite oxide has a composition represented by the following general formula (1):Liα(Ni1−x−yCoxMy)Oβ  (1)
(where x and y each represent an atom ratio, 0.05≦x≦0.35, 0.01≦y≦0.20, and M represents at least one element selected from the group consisting of Mn, Fe, Al, Ga and Mg; and α and β represent an atom ratio provided that the sum of Ni, Co and element M is 1, 0<α<1.1 and 1.9<β<2.1),
the oxygen position parameter (Zo) is 0.2360 to 0.2420, and the lithium-oxygen distance (d) defined by the following formula (2) is 0.2100 nm to 0.2150 nm” (claim 1), and shows that a good initial discharge capacity and charge-discharge cycle characteristics are obtained by preparing such a positive active material that the oxygen position parameter (Zo) and the lithium-oxygen distance (d) have, in a specific composition range, a good correlation with the initial discharge capacity and charge-discharge cycle characteristics, and the oxygen position parameter (Zo) and the lithium-oxygen distance (d) fall within a certain range. (paragraph 0016)